Update legacy-widget-block.md (#44343)

`$control` is not the first argument passed to the event. Anyone who tries to use this code will encounter an error stating `$control.find is not a function`.

This blog post will also need updating: https://make.wordpress.org/core/2021/06/29/block-based-widgets-editor-in-wordpress-5-8/

docs/how-to-guides/widgets/legacy-widget-block.md

@@ -18,7 +18,7 @@ For example, a widget might want to show a "Password" field when the "Change pas
 
 ```js
 ( function ( $ ) {
-	$( document ).on( 'widget-added', function ( $control ) {
+	$( document ).on( 'widget-added', function ( $event, $control ) {
 		$control.find( '.change-password' ).on( 'change', function () {
 			var isChecked = $( this ).prop( 'checked' );
 			$control.find( '.password' ).toggleClass( 'hidden', ! isChecked );